1. Field of the Invention
The present invention relates to an exhaust gas purifying system for an internal combustion engine and, more particularly, to an exhaust gas purifying system provided in an exhaust passage of the internal combustion engine to purify exhaust gas.
2. Description of Related Art
Conventionally, it is known that an exhaust gas purifying system is provided in an exhaust passage of an internal combustion engine to collect particulates (particulate substances) in exhaust gas emitted from the internal combustion engine such as a diesel engine or to reduce NOx content.
As an exhaust gas purifying system for collecting particulates, a system provided with an exhaust emission after-treatment device consisting of a diesel particulate filter (hereinafter referred to as a DPF) has been developed.
As an exhaust gas purifying system for reducing NOx content, a system provided with an exhaust emission after-treatment device consisting of a NOx reduction catalyst (DeNOx catalyst) or a NOx occlusion reduction catalyst has been developed.
In both cases, the exhaust emission after-treatment device of the exhaust gas purifying system uses, for example, a columnar carrier (core) formed of a ceramic material such as cordulite and silicon carbide or a metal. This carrier has a construction such that a large number of small holes are formed in the axial direction in a honeycomb shape.
In the exhaust emission after-treatment device provided with the DPF, the carrier has a function as a filter. Specifically, exhaust gas flows into the carrier from one end face of the carrier, passing through a porous wall (boundary wall) separating the small holes, and flows out of the other end face. When the exhaust gas passes through the wall, particulates in the exhaust gas are collected.
Also, in the exhaust emission after-treatment device provided with the NOx reduction catalyst or the NOx occlusion reduction catalyst, various types of catalysts have been carried in advance in the carrier, and NOx is reduced during the time when exhaust gas flows in the carrier.
Such a carrier has many limitations in manufacturing, so that it is difficult to manufacture a carrier having a remarkably large cross-sectional shape. Therefore, the capacity of the whole carrier must be increased. That is, in order to increase the collecting efficiency of the DPF or to increase the reducing efficiency of the catalyst, a plurality of carriers must be arranged in parallel to increase the capacity of the whole carrier.
However, if a plurality of carriers are arranged in parallel, the cross-sectional area of the whole carrier increases, so that a large space for arranging the carriers must be secured in an engine room, which presents a problem of hindering the downsizing of equipment.
To solve this problem, a system as described below can be thought. In this system, a pair of carriers are arranged in series with a clearance provided therebetween, and exhaust gas is caused to flow into between the carriers, by which a half of the exhaust gas is caused to flow into one carrier and the remaining half of the exhaust gas is caused to flow in the reverse direction so as to flow into the other carrier. Thereby, the capacity of the whole carrier can be doubled without arranging a pair of carriers in parallel.
In this system, however, although a large cross-sectional area is restrained, the flow direction of exhaust gas in each of the carriers is reverse, so that two outlet pipes are needed, and some consideration is still needed for the arrangement space. Therefore, there still remains a problem to be solved.
The main object of the present invention is to provide an exhaust gas purifying system for an internal combustion engine in which the inherent function can be improved by increasing the capacity of the whole of carriers and a large installation space is made unnecessary.
The present invention provides an exhaust gas purifying system for an internal combustion engine, which is provided in an exhaust flow path of the internal combustion engine, including a plurality of carriers for exhaust emission after-treatment arranged in series along the flow direction of exhaust gas; a distribution flow path for distributing exhaust gas to each of the carriers to cause the exhaust gas to flow in the carrier; and a combined flow chamber in which the exhaust gases having passed through the distribution flow paths are combined.
In the above-described exhaust gas purifying system, although the carriers are arranged in series, the exhaust gas passing through a different distribution flow path flows in each of the carriers. Therefore, the capacity of the whole of the carriers increases substantially a plurality of times as in the case of the carriers arranged in parallel, so that the inherent function as an exhaust gas purifying system is improved.
Also, since the exhaust gases having passed through the distribution flow paths are combined in the combined flow chamber, only one outlet pipe communicating with the combined flow chamber has only to be provided. Therefore, an increase in cross-sectional area is restrained because the carriers are arranged in series, and a large installation space is unnecessary because the number of outlet pipes need not be increased.
The present invention provides an exhaust gas purifying system for an internal combustion engine, which is provided in an exhaust flow path of the internal combustion engine, including a plurality of carriers for exhaust emission after-treatment arranged in series along the flow direction of exhaust gas; and a distribution flow path for distributing exhaust gas to each of the carriers to cause the exhaust gas to flow in the carrier; the flow direction of the exhaust gas being set in one direction.
In the above-described exhaust gas purifying system, as in the case of the before-mentioned construction, the capacity of the whole of the carriers increases substantially a plurality of times as in the case of the carriers arranged in parallel, so that the inherent function as an exhaust gas purifying system is improved.
Also, since the flow direction of the exhaust gas flowing in each of the carriers is the same, the exhaust gases having passed through the distribution flow paths are combined easily at one place, so that only one outlet pipe has only to be provided in this combined flow portion. In this case as well, therefore, an increase in cross-sectional area is restrained because the carriers are arranged in series, and a large installation space is unnecessary because the number of outlet pipes need not be increased.
In the present invention, it is preferable that two of the carriers be arranged in series on the upstream side and the downstream side; a bypass flow path be provided on a concentric circle of each of the carriers; between the carriers, there be provided a split flow portion provided with an outlet space in which the exhaust gas having passed through the upstream-side carrier flows, an inlet space in which the exhaust gas to be caused to flow in the downstream-side carrier flows, and a wall portion for partitioning the spaces; a first distribution flow path for the upstream-side carrier be formed so as to include the outlet space of the split flow portion and the downstream-side bypass flow path communicating with the outlet space; and a second distribution flow path for the downstream-side carrier be formed so as to include the upstream-side bypass flow path and the inlet space of the split flow portion communicating with the upstream-side bypass flow path.
According to the above-described exhaust gas purifying system, since two carriers are provided, the capacity is substantially doubled as compared with the case where one carrier is provided.
Further, since the bypass flow path of the first and second distribution flow paths for each carrier is provided on a concentric circle of the carrier, the cross section thereof is formed so as to be annular, fan-shaped, or cylindrical, so that there is no fear of extremely projecting from the carrier. Therefore, the exhaust gas purifying system is formed so as to have a simple outside shape, and also can be made more compact.
In the present invention, it is preferable that there be provided a plurality of carrier arrangement units in which the carrier is arranged individually and a split flow unit provided between the adjacent carrier arrangement units; each of the carrier arrangement units be provided with a bypass flow path on a concentric circle of the carrier; the split flow unit be provided with a split flow portion provided with an outlet space in which the exhaust gas having passed through the upstream-side carrier flows, an inlet space in which the exhaust gas to be caused to flow in the downstream-side carrier flows, and a wall portion for partitioning the spaces; a distribution flow path for the upstream-side carrier be formed so as to include the outlet space of the split flow portion and the downstream-side bypass flow path communicating with the outlet space; and a distribution flow path for the downstream-side carrier be formed so as to include the upstream-side bypass flow path and the inlet space of the split flow portion communicating with the upstream-side bypass flow path.
According to the above-described exhaust gas purifying system, the whole of the system is unitized by the plural carrier arrangement units and the split flow unit. Therefore, the carrier can be replaced easily in a unit, and the handling ability is improved by the interchangeability of carrier arrangement unit. Also, the kinds of members are reduced.
The carrier arrangement unit can be formed so as to be capable of being used by reversing the upstream side and the downstream side. Particularly when the carrier is used as a DPF, the application efficiency is improved by using the carrier arrangement unit by being turned.
In the present invention, it is preferable that the split flow unit have a double tube construction provided with an external cylindrical member and an internal cylindrical member; the internal cylindrical member be provided with at least a pair of opening portions for causing external and internal space portions to communicate with each other; in the internal cylindrical member, an internal wall for separating the paired opening portions be provided; between the external cylindrical member and the internal cylindrical member, an external wall for separating the paired opening portions be provided; the outlet space be formed by the external and internal space portions of the internal cylindrical member which are caused to communicate with each other by either one of the paired opening portions; the inlet space be formed by the external and internal space portions of the internal cylindrical member which are caused to communicate with each other by the other one of the paired opening portions; and the wall portion be formed by the internal wall and the external wall.
According to the above-described exhaust gas purifying system, by merely using the split flow unit, the exhaust gas having flowed in the upstream-side carrier passes through the outlet space having the opening portion and flows in the downstream-side bypass flow path, and the exhaust gas having flowed in the upstream-side bypass flow path passes through the inlet space having the opening portion and flows in the downstream-side carrier. Therefore, the distribution flow path for each carrier is formed easily.
In the present invention, it is preferable that the internal wall be tilted with respect to the flow direction of the exhaust gas in the carrier; and the opening portion be open along the peripheral edge of the internal wall.
According to the above-described exhaust gas purifying system, since the internal wall is tilted, in the outlet space, the exhaust gas having passed through the upstream-side carrier is introduced smoothly to the opening portion along the tilted internal wall, so that the exhaust gas is discharged efficiently.
Also, in the inlet space, the exhaust gas entering the inlet space through the opening portion collides with the tilted internal wall and hence the flow thereof is straightened. Therefore, the flow distribution is improved, and then the exhaust gas flows into the downstream-side carrier, so that the tilted surface of the internal wall can also be used as a flow straightening device.
In the present invention, it is preferable that a flow straightening device for straightening the flow of exhaust gas flowing in the carrier be provided on the upstream side of each of the carriers.
According to the above-described exhaust gas purifying system, since the flow distribution of the exhaust gas flowing into the carrier is improved, there is no fear of exhaust gas flowing concentratedly into a part of carrier. Therefore, when the carrier is used by carrying a catalyst therein, only a part of the catalyst is not exposed to exhaust gas concentratedly, so that the catalytic action can be accomplished efficiently. Also, when the carrier is used as a DPF, the carrier is not clogged with particulates non-uniformly, so that the temperature distribution at the time of recycling the carrier is uniformized, by which the breakage of the carrier caused by the thermal stress is prevented.
In the present invention, it is preferable that an inlet pipe for causing exhaust gas to flow into the exhaust gas purifying system and an outlet pipe for discharging exhaust gas from the exhaust gas purifying system be installed substantially at right angles with the flow direction of exhaust gas in the carrier.
According to the above-described exhaust gas purifying system, since the inlet pipe and the outlet pipe are installed substantially at right angles with the flow direction of exhaust gas in the carrier, the layout of the inlet pipe and the outlet pipe is accomplished easily. Therefore, the exhaust gas purifying system can be made more compact, and can be installed in a smaller installation space.